colibri_t20: enable ubi/ubifs support

Now as all is in place actually enable UBI/UBIFS support to be able to
boot the Linux kernel and root file system from NAND.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>